0620 · 7.2Oxides

Classifying basic oxides

0:00Carbon dioxide, copper(II) oxide and aluminium oxide all contain oxygen, but their reactions are very different. One reacts with bases, another neutralises acids, and the third can do both. Their names alone do not explain this behaviour. By the end of this video, you will understand how those reactions help us classify oxides. An oxide is a compound of oxygen and another element. Carbon dioxide contains carbon and oxygen, while calcium oxide contains calcium and oxygen.

0:29The important difference is that carbon is a non-metal, whereas calcium is a metal. This distinction gives us a useful starting point for predicting the behaviour of their oxides. We also need the idea of neutralisation. An acid reacts with a base to produce a salt and water. A base does not have to dissolve in water to react with an acid. An alkali is a soluble base, so the word base covers more substances than the word alkali.

0:54Metal oxides are generally basic, while non-metal oxides are generally acidic. Notice the word generally, because this pattern has exceptions. Calcium oxide and copper(II) oxide are basic oxides. Because carbon and sulfur have non-metallic character, their dioxides are both classified as acidic. These descriptions tell us about chemical behaviour, not simply appearance or physical state. A basic oxide reacts with an acid to produce a salt and water. With a base, an acidic oxide gives the same two kinds of product.

1:23We classify the substance by its reactions, rather than whether it is a solid or a gas. Copper(II) oxide is a good first example. It reacts with hydrochloric acid to produce copper(II) chloride and water. The oxide acts as a base because it neutralises the acid. Copper(II) chloride is the salt formed in this particular reaction. There is a useful particle explanation behind this reaction. Copper(II) oxide contains oxide ions, which have a 2− charge.

1:51Hydrogen ions from the acid combine with those oxide ions to form water. The copper(II) ions and chloride ions remain in solution, giving the dissolved salt. This explains why containing oxygen is not enough to make a substance acidic. In copper(II) oxide, the oxide ions can accept hydrogen ions from an acid. That is basic behaviour. Calcium oxide also reacts with acids to produce a salt and water, so it belongs in the same category.

2:17Do not confuse a basic oxide with an alkali. Copper(II) oxide is insoluble in water, but it still reacts with acids. Its insolubility does not prevent neutralisation at the surface of the solid. Basic behaviour is therefore not restricted to substances that form alkaline solutions.

Acidic oxides and equations

2:33Carbon dioxide shows the opposite behaviour. It reacts with the base calcium hydroxide to produce calcium carbonate and water. Calcium carbonate is the salt in this reaction. Because carbon dioxide neutralises a base, we classify it as an acidic oxide. Sulfur dioxide is another acidic oxide that you need to recognise. Both examples contain a non-metal combined with oxygen. Their reactions with bases distinguish them from calcium oxide and copper(II) oxide.

2:59The metallic or non-metallic character of the other element helps predict which behaviour to expect. Some acidic oxides also form acidic solutions when they dissolve in water. However, reaction with water is not the definition we should rely on here. The key classification evidence is their reaction with bases. Likewise, a basic oxide does not have to dissolve in water before it can neutralise an acid. Predicting products is a skill the exam tests directly.

3:26Suppose we are asked to state the products when copper(II) oxide reacts with hydrochloric acid. First, identify the oxide as basic. Its reaction with an acid must therefore produce a salt and water. Next, determine the salt name from the reactants. Copper(II) oxide supplies the copper, and hydrochloric acid supplies chloride, so the salt is copper(II) chloride. The complete word equation is copper(II) oxide plus hydrochloric acid gives copper(II) chloride plus water.

3:53The balanced symbol equation is on the board. The coefficient of two before hydrochloric acid balances both hydrogen and chlorine. Two hydrogen atoms are needed to form each water molecule. The salt also contains two chloride ions for each copper(II) ion. Checking both elements explains the coefficient, rather than treating it as a number to memorise.

Amphoteric oxides

4:13This next part is for the Extended paper. Amphoteric oxides are oxides that react with acids and with bases to produce a salt and water. Aluminium oxide and zinc oxide are the two examples you need to classify as amphoteric. Both contain metals, so they are exceptions to the simple metal oxide pattern. Think of an amphoteric oxide as having two possible reaction roles. When reacting with an acid, it behaves as a base.

4:37With a base, it shows acidic behaviour instead. This does not mean it is a mixture of separate acidic and basic substances. The same compound can participate in either type of reaction, depending on its reaction partner. Aluminium oxide can neutralise an acid, but it can also react with a base. Zinc oxide behaves in the same broad way. That additional reaction with bases is what distinguishes these examples from ordinary basic oxides.

5:03A reaction with an acid alone cannot prove that an oxide is amphoteric. Ordinary basic oxides also produce a salt and water in that situation. To identify amphoteric behaviour, you need evidence of reaction with a base as well. Both kinds of reaction must produce a salt and water. An exam question might describe an oxide that reacts with hydrochloric acid and sodium hydroxide to produce a salt and water.

5:25It could ask you to state its classification and explain your answer. Hydrochloric acid is the acid in this question, while sodium hydroxide acts as the base. Together, these results identify amphoteric behaviour. A precise answer would be that the oxide is amphoteric. It reacts with acids and also with bases, and each reaction produces a salt and water. If asked to state one possible identity, aluminium oxide or zinc oxide would be suitable.

5:52Examiners often see three classification mistakes. Do not label every metal oxide as basic, because aluminium oxide and zinc oxide are amphoteric. Also, insolubility does not mean an oxide cannot behave as a base. Finally, forming a salt is not enough detail when describing amphoteric reactions; include water and both types of reaction partner. The central idea is that oxide classification describes reactions. Carbon dioxide and sulfur dioxide are acidic oxides, linked to the non-metallic character of carbon and sulfur.

6:24The metallic character of copper and calcium makes their oxides basic. Aluminium oxide and zinc oxide are amphoteric because they react with both acids and bases to produce a salt and water.